The technology disclosed in U.S. Pat. No. 7,237,315 issued Jul. 3, 2007 and entitled “Method for Fabricating a Resonator” is improved upon by this invention. Both the prior art and this invention use similar MEMS fabrication technology to form a quartz resonator structure. However, the technology disclosed in U.S. Pat. No. 7,237,315 really works best when used to make resonators which operate at frequencies at the upper end of the UHF band or even higher.
But there is a need for quartz resonators which operate at even lower frequencies (less than 50 MHz, for example). A problem arises when using the technology of U.S. Pat. No. 7,237,315 to try to make lower frequency resonators—the thickness of the quartz resonator must be increased, but due to the vastly different quartz thicknesses between the higher end of the UHF band in one hand and lower frequency devices on the other hand (several microns of quartz thickness for the upper UHF frequency devices compared with several tens or hundreds of microns thickness for lower frequency devices), the soft photoresist mask used in U.S. Pat. No. 7,237,315 cannot be successfully utilized. In U.S. Pat. No. 7,237,315 the photoresist is the “soft” mask which is used with plasma dry etching. In this disclosure a “hard” mask is used instead because the presently disclosed method uses a wet etchant at an elevated temperature for typically many hours. A “soft” mask can not withstand such an aggressive wet etch and therefore a Cr/Au (metal hard) mask is suggested herein. The soft masks can be seen used in FIGS. 1f and 1h of the prior art which are used for etching the via and the resonator.
This invention also introduces a novel quartz resonator temporary attachment and release technology that can increase device yield and lower cost. In U.S. Pat. No. 7,237,315 silicon or GaAs were used as the handle wafer because the quartz resonator wafer was bonded to the handle using a room temperature direct bond without any adhesives. The handle can be dissolved or etched away later using a preferential etch that does not attack quartz. As long as you can find a material that can be directly bonded to quartz at room temperature and preferentially removed later, you can use it.
In this disclosure, a quartz handle is suggested, which is inconsistent with the prior art because with the prior art direct bonding process the quartz handle can not be preferentially removed without also attacking to the quartz resonator wafer.
Also not having to form the cavity in the handle as done in the prior art is an improvement for the quartz handle in that putting a cavity into a quartz substrate can be omitted. The adhesive bond to the handle is a high temperature bond that occurs at 150° C. (for example).
The process flow from the above-identified US patent is shown, in simplified form, by FIGS. 1a-1k. The process begins by providing a quartz substrate 2 having a first surface 3 and a second surface 5, a silicon or GaAs handle substrate 4, and a base or host substrate 14. A portion of the silicon handle substrate 4 is etched away creating a cavity 6, as shown in FIG. 1b. The etched cavity 6 can be fabricated with a wet etch of potassium hydroxide, or a dry reactive ion etch using a gas having a fluorine chemistry. Then, top-side electrode and tuning pad metal (Al or Au) 7 is deposited onto a quartz substrate 2 as shown by FIG. 1c. Next, the two wafers 2, 4 are brought together using a direct bonding process as depicted by FIG. 1d. After a low temperature bonding/annealing operation, a combination of processes including wafer grinding/lapping, chemical-mechanical-planarization (CMP), plasma etching and chemical polishing is used to thin the quartz down to a thickness, typically less than 10 microns, for a desired resonant frequency as depicted by FIG. 1e. Next, photolithography is used to pattern contact via holes in the thinned quartz layer 2. The holes are etched through quartz to stop on top-side electrode metal 7 and then metallized to form the through-wafer metal vias 11 as shown in FIG. 1f. The bottom-side electrodes 12 are then metallized (see FIG. 1g), and the quartz layer is patterned and etched (see FIG. 1h) to form an array of resonators. Finally, protrusions are etched into the host substrate 14, and metalization patterns 16, including bonding pads, are defined on the substrate 14 as depicted by FIG. 1i. The quartz/silicon pair 2,4 is bonded to the host wafer 14 using either a Au—Au or Au—In compression bonding scheme (see FIG. 1j), and the silicon handle wafer 4 is thereafter removed with a combination of dry and wet etches, resulting in the quartz resonators being attached only to the host wafer, as shown in FIG. 1k. The prior art uses a spin coating of a soft mask (photoresist) for patterning of the metal, quartz and silicon structures.